Process for the production of an olefin oxide

ABSTRACT

The invention provides a process for the epoxidation of an olefin, which process comprises reacting a feed comprising an olefin and oxygen in the presence of a catalyst comprising a carrier and silver deposited on the carrier, which carrier comprises at least 85 weight percent α-alumina and has a surface area of at least 1.3 m 2 /g, a median pore diameter of more than 0.8 μm, and a pore size distribution wherein at least 80% of the total pore volume is contained in pores with diameters in the range of from 0.1 to 10 μm and at least 80% of the pore volume contained in the pores with diameters in the range of from 0.1 to 10 μm is contained in pores with diameters in the range of from 0.3 to 10 μm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 11/921,610filed Dec. 5, 2007, and claims the benefit of International ApplicationNo. PCT/US2006/021903, filed Jun. 6, 2006, which designated the U.S. andwhich claims priority to U.S. Patent Application No. 60/688,245, filedJun. 7, 2005, with the U.S. Patent and Trademark Office. The entiredisclosures of these applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a catalyst, a process for preparing thecatalyst, and a process for the production of an olefin oxide, a1,2-diol, a 1,2-diol ether, or an alkanolamine.

BACKGROUND OF THE INVENTION

In olefin epoxidation, a feed containing an olefin and an oxygen sourceis contacted with a catalyst under epoxidation conditions. The olefin isreacted with oxygen to form an olefin oxide. A product mix results thatcontains olefin oxide and typically unreacted feed and combustionproducts.

The olefin oxide may be reacted with water to form a 1,2-diol, with analcohol to form a 1,2-diol ether, or with an amine to form analkanolamine. Thus, 1,2-diols, 1,2-diol ethers, and alkanolamines may beproduced in a multi-step process initially comprising olefin epoxidationand then the conversion of the formed olefin oxide with water, analcohol, or an amine.

Olefin epoxidation catalysts comprise a silver component, usually withone or more additional elements deposited therewith, on a carrier.Carriers are typically formed of a refractory material, such asalpha-alumina. In general, higher purity alpha-alumina has been found tocorrelate with better performance. It has also been found for examplethat the presence of minor amounts of impurities in the carrier such asalkali and/or alkaline earth metals and some forms of silica can have abeneficial effect.

Intuitively it might also be considered that the higher the surface areaof the carrier, the greater the area available for deposition of thesilver and therefore the more effective the silver deposited thereon.However, this is generally found not to be the case and in moderncatalysts the tendency is to use a carrier with a relatively low surfacearea, for example a surface area of less than 1.3 m²/g, or even lessthan 1 m²/g.

US 2003/0162984 A1 discloses carriers which have a surface area of atleast 1 m²/g. The working examples given show improved initialselectivity and activity of epoxidation catalysts based on carriershaving at least 70% of the total pore volume represented by pores withdiameters in the range of from 0.2 to 10 μm.

The catalyst performance may be assessed on the basis of selectivity,activity and stability of operation. The selectivity is the fraction ofthe converted olefin yielding the desired olefin oxide. As the catalystages, the fraction of the olefin converted normally decreases with timeand to maintain a constant level of olefin oxide production thetemperature of the reaction is increased. However this adversely affectsthe selectivity of the conversion to the desired olefin oxide. Inaddition, the equipment used can tolerate temperatures only up to acertain level so that it is necessary to terminate the reaction when thereaction temperature would reach a level inappropriate for the reactor.Thus the longer the selectivity can be maintained at a high level andthe epoxidation can be performed at an acceptably low temperature, thelonger the catalyst charge can be kept in the reactor and the moreproduct is obtained. Quite modest improvements in the maintenance ofselectivity over long periods yields huge dividends in terms of processefficiency.

SUMMARY OF THE INVENTION

The present invention provides a catalyst which comprises a carrier andsilver deposited on the carrier, which carrier has a surface area of atleast 1 m²/g, and a pore size distribution wherein at least 80% of thetotal pore volume is contained in pores with diameters in the range offrom 0.1 to 10 μm and at least 80% of the pore volume contained in thepores with diameters in the range of from 0.1 to 10 μm is contained inpores with diameters in the range of from 0.3 to 10 μm.

The invention also provides a process for the preparation of a catalystwhich process comprises:

a) selecting a carrier which has a surface area of at least 1 m²/g, anda pore size distribution wherein at least 80% of the total pore volumeis contained in pores with diameters in the range of from 0.1 to 10 μmand at least 80% of the pore volume contained in the pores withdiameters in the range of from 0.1 to 10 μm is contained in pores withdiameters in the range of from 0.3 to 10 μm, andb) depositing silver on the carrier.

The invention also provides a process for the preparation of a catalystwhich process comprises depositing silver on a carrier, wherein thecarrier has been obtained by a method which comprises forming a mixturecomprising:

a) from 50 to 95 weight percent of a first particulate α-alumina havinga median particle size (d₅₀) of from 5 to 100 μm;

b) from 5 to 50 weight percent of a second particulate α-alumina havinga d₅₀ which is less than the d₅₀ of the first particulate α-alumina andwhich is in the range of from 1 to 10 μm; and

c) an alkaline earth metal silicate bond material;

weight percent being based on the total weight of α-alumina in themixture; and firing the mixture to form the carrier.

Further, the invention provides a process for the epoxidation of anolefin, which process comprises reacting a feed comprising an olefin andoxygen in the presence of a catalyst which comprises a carrier andsilver deposited on the carrier, which carrier has a surface area of atleast 1 m²/g, and a pore size distribution wherein at least 80% of thetotal pore volume is contained in pores with diameters in the range offrom 0.1 to 10 μm and at least 80% of the pore volume contained in thepores with diameters in the range of from 0.1 to 10 μm is contained inpores with diameters in the range of from 0.3 to 10 μm.

The invention also provides a process for the epoxidation of an olefin,which process comprises reacting a feed comprising an olefin and oxygenin the presence of a catalyst which has been obtained by a process whichcomprises depositing silver on a carrier, wherein the carrier has beenobtained by a method which comprises forming a mixture comprising:

a) from 50 to 95 weight percent of a first particulate α-alumina havinga d₅₀ of from 5 to 100 μm;

b) from 5 to 50 weight percent of a second particulate α-alumina havinga d₅₀ which is less than the d₅₀ of the first particulate α-alumina andwhich is in the range of from 1 to 10 μm; and

c) an alkaline earth metal silicate bond material;

weight percent being based on the total weight of α-alumina in themixture; and firing the mixture to form the carrier.

The invention also provides a process for preparing a 1,2-diol, a1,2-diol ether or an alkanolamine comprising converting the olefin oxideinto the 1,2-diol, the 1,2-diol ether or the alkanolamine wherein theolefin oxide has been obtained by a process for the epoxidation of anolefin in accordance with this invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the teaching of this invention, by maximizing forcertain high-surface area carriers the number of pores having a diameterin the range of 0.3 to 10 μm, in particular by minimizing the porevolume in pores having diameters less than 0.3 μm, the catalyst based onthe carrier is advantaged over catalysts that are prepared from carrierswhich have a substantial pore volume in pores having diameters less than0.3 μm. In particular, catalysts prepared according to this inventionshow excellent activity and selectivity, and they are believed toprovide significant improvements in stability under conditions ofcommercial operation. This is non-obvious in view of the prior artacknowledged hereinbefore. US 2003/0162984 A1 teaches improvedperformance of catalysts based on carriers having at least 1 m²/gsurface area and having at least 70% of the total pore volume containedin pore with diameters in the range of from 0.2 to 10 μm. The teachingof US 2003/0162984 is such that a skilled person would utilize carrierswith, in particular, a minimized number of pores having diametersgreater than 10 μm. The reference does not contain teaching relevant tothe pore size distribution within the range of pore diameters from 0.2to 10 μm, and it does not contain teaching relevant to the stability ofthe catalysts, for example, under conditions of commercial operation.

“Surface area” as used herein is understood to refer to the surface areaas determined by the nitrogen BET (Brunauer, Emmett and Teller) methodas described in Journal of the American Chemical Society 60 (1938) pp.309-316.

As used herein, water absorption is deemed to have been measured inaccordance with ASTM C393, and water absorption is expressed as theweight of the water that can be absorbed into the pores of the carrier,relative to the weight of the carrier.

The pore size distribution may be measured by a conventional mercuryintrusion porosimetry device in which liquid mercury is forced into thepores of a carrier. Greater pressure is needed to force the mercury intothe smaller pores and the measurement of pressure increments correspondsto volume increments in the pores penetrated and hence to the size ofthe pores in the incremental volume. As used herein, the pore sizedistribution, the median pore diameters and the pore volumes are asmeasured by mercury intrusion porosimetry to a pressure of 2.1×10⁸ Pausing a Micromeretics Autopore 9200 model (130° contact angle, mercurywith a surface tension of 0.480 N/m, and correction for mercurycompression applied). As used herein, the median pore diameter is thepore diameter at which half of the total pore volume is contained inpores having a larger pore diameter and half of the total pore volume iscontained in pores having a smaller pore diameter.

The median particle size, referred to herein as “d₅₀”, is as measured bya Horiba LA900 particle size analyzer and represents a particle diameterat which there are equal spherical equivalent volumes of particleslarger and particles smaller than the stated median particle size. Themethod includes dispersing the particles by ultrasonic treatment, thusbreaking up secondary particles into primary particles. Thissonification treatment is continued until no further change in the d₅₀value is noticed, which typically requires a 5 minute sonification whenusing the Horiba LA900 particle size analyzer.

As used herein, pore volume (ml/g), surface area (m²/g) and waterabsorption (g/g) are defined relative to the weight of the carrier,unless stated otherwise.

In accordance with this invention, a carrier is used which has a poresize distribution such that at least 80% of the total pore volume iscontained in pores with diameters in the range of from 0.1 to 10 μm, andat least 80% of the pore volume contained in the pores with diameters inthe range of from 0.1 to 10 μm is contained in pores with diameters inthe range of from 0.3 to 10 μm. Preferably, the pore size distributionis such that the pores with diameters in the range of from 0.1 to 10 μmrepresent at least 85%, in particular at least 90%, more preferably atleast 95% of the total pore volume. Typically, the pore sizedistribution is such that pores with diameters less than 0.1 μmrepresent less than 10%, more typically at most 7%, in particular atmost 5%, more in particular at most 1%, or even at most 0.5% or at most0.1% of the total pore volume. Typically, the pore size distribution issuch that pores with diameters greater than 10 μm represent less than10%, in particular at most 8%, more in particular at most 6%, of thetotal pore volume.

Frequently, the pore size distribution is such that the pores withdiameters in the range of from 0.1 to 10 μm represent less than 99.9%,more frequently less than 99%, most frequently less than 98% of thetotal pore volume. Frequently, the pores with diameters greater than 10μm represent more than 0.1%, more frequently more than 0.5% of the totalpore volume. The invention contemplates pores with diameters less than0.1 μm approaching, if not reaching, zero percent of the total porevolume.

Typically, the pore size distribution is such that the pores withdiameters in the range of from 0.3 to 10 μm represent at least 85%, inparticular at least 90%, more in particular at least 95% of the porevolume contained in the pores with diameters in the range of from 0.1 to10 μm.

Typically, the pore size distribution is such that pores with diametersless than 0.3 μm represent less than 15%, more typically at most 10%, inparticular at most 5%, more in particular at most 3% of the total porevolume. Frequently, the pore size distribution is such that pores withdiameters less than 0.3 μm represent more than 0.01%, more frequentlymore than 0.1% of the total pore volume.

In another embodiment, the pore size distribution is such that the poreswith diameters in the range of from 0.4 to 10 μm represent at least 75%,in particular at least 80% of the pore volume contained in the poreswith diameters in the range of from 0.1 to 10 μm. In another embodiment,the pore size distribution is such that the pores with diameters in therange of from 0.5 to 10 μm represent at least 60%, in particular atleast 65% of the pore volume contained in the pores with diameters inthe range of from 0.1 to 10 μm.

In another embodiment, the pore size distribution is such that the poreswith diameters in the range of from 2 to 10 μm represent at least 20%,more typically at least 30%, in particular at least 40% of the porevolume contained in the pores with diameters ranging from 0.1 to 10 μm.In another embodiment, the pore size distribution is such that the poreswith diameters in the range of from 5 to 10 μm represent at least 15%,more typically at least 20% of the pore volume contained in the poreswith diameters ranging from 0.1 to 10 μm.

The carriers may have a median pore diameter of more than 0.8 μm,preferably at least 0.85 μm, more preferably at least 0.9 μm. Typically,the median pore diameter is at most 2.1 μm, more typically at most 2 μm,in particular at most 1.9 μm. Preferably, the median pore diameter is inthe range of from 0.85 to 1.9 μm, more preferably in the range of from0.9 to 1.8 μm.

The total pore volume of the carrier may vary between wide ranges.Typically the total pore volume is at least 0.25 ml/g, in particular atleast 0.3 ml/g, more in particular at least 0.35 ml/g. Typically, thetotal pore volume is at most 0.8 ml/g, and more typically it is at most0.7 ml/g, in particular at most 0.6 ml/g.

The surface area of the carrier may be at least 1.3 m²/g. Typically, thesurface area is at most 5 m²/g. Preferably, the surface area is in therange of from 1.3 to 3 m²/g, more preferably from 1.4 to 2.5 m²/g, mostpreferably from 1.5 to 2.2 m²/g, for example from 1.5 to 2 m²/g.

The water absorption of the carrier is typically at least 0.3 g/g, moretypically at least 0.35 g/g. Frequently, the water absorption is at most0.8 g/g, more frequently at most 0.7 g/g, or at most 0.6 g/g, or at most0.55 g/g. Preferably, the water absorption of the carrier is in therange of from 0.3 to 0.7 g/g, in particular from 0.35 to 0.55 g/g. Ahigher water absorption and a higher total pore volume are in favor inview of a more efficient deposition of silver and further elements, ifany, on the carrier by impregnation. However, at a higher waterabsorption and higher total pore volume, the carrier, or the catalystmade therefrom, may have lower crush strength.

In certain embodiments of this invention, the carrier exhibits anon-platelet morphology. As used herein, the term “non-plateletmorphology” refers to the morphology of the carrier when imaged byscanning electron microscopy at a magnification of 2000, and to thesubstantial absence in such images of structures having substantiallyflat surfaces. By “substantial absence” of such structures it is meantthat at most 25% of the structures have a substantially flat surface. By“substantially flat” it is meant that the radius of the curvature of thesurface is at least 2 times the length of the largest dimension of thesurface. The structures having a substantially flat surface havetypically an aspect ratio of at most 4:1, the aspect ratio of astructure being the ratio of the largest dimension to the smallestdimension of the structure. The term “structures” refers to structuralentities in the carrier which can be designated to represent individualparticles of carrier material fused or bonded together to form thecarrier.

The carrier may be based on a wide range of materials. Such materialsmay be natural or artificial inorganic materials and they may includerefractory materials, silicon carbide, clays, zeolites, charcoal andalkaline earth metal carbonates, for example calcium carbonate.Preferred are refractory materials, such as alumina, magnesia, zirconiaand silica. The most preferred material is α-alumina. Typically, thecarrier comprises at least 85 weight percent, more typically 90 weightpercent, in particular 95 weight percent α-alumina, frequently up to99.9 weight percent α-alumina.

Carriers may generally be made by firing particulate components at anelevated temperature until the particles sinter together. In general;firing may be continued until the particles are bonded together, eitherby the formation of bond posts from any added bond material or throughsintering, but preferably not beyond the point at which the waterabsorption of the carrier is reduced.

Burnout materials may or may not be used in the firing process. Burnoutmaterials are well known in the art (cf., for example, F F Y Wang (Ed.),“Treatise on Materials Science and Technology”, Volume 9, (New York,1976), pp. 79-81; or J S Reed, “Introduction to the Principles ofCeramic Processing”, (New York, 1988), pp. 152 ff.). The burnoutmaterials may be used to enhance preservation of the structure during agreen, i.e. unfired, phase of the carrier preparation, for example thephase in which formed bodies are shaped, for example by extrusion. Theburnout materials are removed during the firing. The use of burnoutmaterials also allows more complete sintering without too great areduction in water absorption of the carrier. The burnout materials aretypically finely divided solid organic materials that volatilize orburn, leaving as little residue as possible.

It is also a common expedient to use a bond material, i.e. a materialwhich reduces the length of sintering time applied to bond the particlestogether. The bond material may also form a coating on at least a partof the carrier surface, which makes the carrier surface more receptive.The bond material may be based on a silica-containing compositioncomprising a crystallization inhibitor, inhibiting the formation ofcrystalline silica-containing compositions.

The silica-containing compositions for use as a bond material maycomprise an alkali metal silicate bond material, or preferably analkaline earth metal silicate bond material. The bond material mayfurther comprise a hydrated alumina and optionally a titanium componentand/or a zirconium component.

It has been found that, suitably, alumina carriers for use in thisinvention may be made by a method which comprises forming a mixturecomprising:

a) from 50 to 95 weight percent of a first particulate α-alumina havinga d₅₀ of from 5 to 100 μm, in particular from 8 to 60 μm, more inparticular from 10 to 40 μm;

b) from 5 to 50 weight percent of a second particulate α-alumina havinga d₅₀ which is less than the d₅₀ of the first particulate α-alumina andwhich is in the range of from 1 to 10 μm, in particular from 2 to 8 μm;and preferably in addition

c) an alkaline earth metal silicate bond material;

weight percent being based on the total weight of α-alumina in themixture; and then shaping the mixture into formed bodies and firing theformed bodies, typically at a temperature of from 1250 to 1550° C., toform the carrier.

The present method for making alumina carriers is well adapted toproduce the carriers for use in this invention, in view of the carefulmatching of large and small particles of the α-alumina components. Thealumina particles are readily commercially available, or they mayreadily be made, for example, by subjecting more coarse materials togrinding and sieving operations. In an embodiment, the smaller particlesmay be prepared from the larger particles by grinding, and the groundand un-ground particles are then combined. In another embodiment, thedesired mixture of large and small particles may be formed by grindingrelatively large particles to the extent that the mixture of particleshas the desired bimodal particle size distribution.

Typically, the first particulate α-alumina is employed in a quantity offrom 60 to 90 weight percent, relative to the total weight of α-aluminain the mixture. Typically, the second particulate α-alumina is employedin a quantity of from 10 to 40 weight percent, relative to the totalweight of α-alumina in the mixture.

In one embodiment, a carrier of this invention can be made using aluminapowders, designated above as the “first particulate” and the “secondparticulate”, that are characterized as follows. The first particulatepowder has a BET surface area of 4.3 m²/g, a d₅₀ median particle size of15 μm and a pore size distribution wherein pores having diameters lessthan 0.3 μm, and preferably less than 0.2 μm, contribute less than 4percent of the first particulate powder's total pore volume. The secondparticulate powder has a surface area of 1.4 m²/g, a d₅₀ median particlesize of 3.1 μm and a pore size distribution wherein pores havingdiameters less than 0.3 μm, and preferably less than 0.2 μm, contributeless than 1 percent of the second particulate powder's total porevolume. The first and second powders' pore size distributions and porevolumes can be measured by mercury intrusion porosimetry beginning at2413 Pa and then increased to 4.1×10⁷ Pa using a Micromeretics Model9520 Autopore IV (130° contact angle, mercury with a surface tension of0.48 m²/g, and correction for mercury compression applied). The aluminapowders selected for use in making a carrier can impact the physicalcharacteristics, such as pore size distribution and total pore volume,of the carrier. Reducing the percentage of the first and second aluminapowders' pore volumes contributed by pores less than 0.3 μm is believedto result in a carrier with a minimum quantity of its total pore volumecontributed by small pores.

The alkaline earth metal silicate bond material may comprise an alkalineearth metal silicate, for example calcium silicate or, preferably,magnesium silicate. Alternatively to or in addition to the alkalineearth metal silicate, the alkaline earth metal silicate bond materialmay comprise a combination of an alkaline earth metal compound and asilica compound. In such combination the atomic ratio of the alkalineearth metal to silicon is typically in the range of from 0.5 to 2, moretypically 0.8 to 1.4 and most typically 0.9 to 1.2. Suitable alkalineearth metal compounds are alkaline earth metal salts, for examplenitrates or sulfates, in particular magnesium nitrate or magnesiumsulfate. Suitable silica compounds are silica sol, precipitated silica,amorphous silica, amorphous alkali metal silica, or amorphous alkalimetal aluminosilicate. Amorphous silica compounds are preferred. Thequantity of alkaline earth metal silicate bond material may suitably bein the range of from 0.2 to 10 weight percent, more suitably from 0.2 to2 weight percent, in particular from 0.5 to 2 weight percent, calculatedas the total weight of alkaline earth metal oxide and silicate, as SiO₂,relative to the total weight of α-alumina in the mixture.

The alkaline earth metal silicate bond material may or may not comprise,as an additional component, a hydrated alumina. A suitable hydratedalumina is, for example, gibbsite, bayerite or diaspore. A preferredhydrated alumina is boehmite. The quantity of the hydrated alumina maysuitably be in the range of from 0.1 to 15 weight percent, from 0.2 to10 weight percent, or from 0.5 to 5 weight percent, calculated as theweight of aluminium oxide, Al₂O₃, relative to the total weight ofα-alumina in the mixture.

The alkaline earth metal silicate bond material may or may not comprise,as an additional component, a zirconium component, as a solid componentor as a liquid component. Suitable zirconium components are zirconiumdioxide and zirconium compounds which convert to zirconium dioxide uponfiring. Such zirconium compounds may be salts, such as zirconyl nitrate,zirconyl sulfate or basic zirconyl carbonate. The quantity of thezirconium component may suitably be in the range of from 0 to 10 weightpercent, more suitably from 0.2 to 5 weight percent, calculated as theweight of zirconium dioxide, ZrO₂, relative to the total weight ofα-alumina in the mixture.

The alkaline earth metal silicate bond material may or may not comprise,as an additional component, a titanium component. Suitable titaniumcomponents are titanium dioxide, titanyl sulfate, titanyl oxalate,titanyl chloride, organo titanates, and other compounds which convert totitanium dioxide upon firing. Hydrated aluminas may in some instances becontaminated with titanium compounds and act as a source of the titaniumcomponent. The quantity of the titanium component may suitably be in therange of from 0 to 5 weight percent, more suitably from 0 to 1 weightpercent, even more suitably from 0.01 to 0.5 weight percent, inparticular from 0.1 to 0.3 weight percent, calculated as the weight oftitanium dioxide, TiO₂, relative to the total weight of α-alumina in themixture.

In an embodiment, the alkali metal silicate bond material may comprisean alkali metal silicate, for example amorphous sodium or lithiumsilicate.

Burnout materials may be selected from the group of polypropylenes,polyethylenes, carbohydrates, gums, flours, proteins, lignins, resins,waxes, alcohols, and esters. When preparing an α-alumina carrier, thequantity of burnout material may suitably be in the range of from 0.2 to10 weight percent, more suitably from 0.5 to 5 weight percent, relativeto the total weight of α-alumina in the mixture. The selection of theburnout material is considered not to be of any criticality to theinvention. Also, in the practice of this invention using an α-aluminacarrier, no burnout material may be used in the preparation of thecarrier.

It is also preferred that the carrier particles be prepared in the formof formed bodies, the size of which is in general determined by thedimensions of an epoxidation reactor in which they are to be deposited.Generally however it is found very convenient to use particles such asformed bodies in the form of powder, trapezoidal bodies, cylinders,saddles, spheres, doughnuts, and the like. The cylinders may be solid orhollow, straight or bent, and they may have their length andcross-sectional dimensions about the same and from 5 to 10 mm.

The formed bodies can be formed from the mixture by any convenientforming process, such as spraying, spray drying, agglomeration orpressing, but preferably they are formed by extrusion of the mixture.For applicable methods, reference may be made to, for example, U.S. Pat.No. 5,145,824, U.S. Pat. No. 5,512,530, U.S. Pat. No. 5,384,302, U.S.Pat. No. 5,100,859 and U.S. Pat. No. 5,733,842, which are hereinincorporated by reference. To facilitate such molding processes, inparticular extrusion, the mixture may suitably be compounded with up toabout 30 weight percent and preferably from 2 to 25 weight percent,based on the weight of the mixture, of extrusion aids and/or organicbinders. Extrusion aids (also referred to by the term “processing aids”)and organic binders are known in the art (cf., for example, “Kirk-OthmerEncyclopedia of Chemical Technology”, 4^(th) edition, Volume 5, pp. 610ff.). Suitable examples may be petroleum jelly, hydrogenated oil,synthetic alcohol, synthetic ester, glycol, starch, polyolefin oxide orpolyethylene glycol. Boric acid may also be added to the mixture, forexample in a quantity of up to 0.5 weight percent, more typically in aquantity of from 0.01 to 0.5 weight percent, based on the weight of themixture. The effect of the presence of boric acid may be a reducedcontent of leachable alkali metal ions in the carrier after firing.Enough water may be added to the mixture to make the mixture extrudable(by the term “the weight of the mixture”, as used hereinbefore, is meantthe weight of the total mixture, but excluding the weight of any addedwater).

The formed bodies may be dried and fired at a temperature high enough toensure that the alumina particles are joined together by a sinteringaction and/or by the formation of bond posts formed from the bondmaterial, if incorporated in the mixture. Generally, drying may takeplace between 20 and 400° C. and preferably between 30 and 300° C.,typically for a period of up to 100 hours and preferably from 5 minutesto 50 hours. Typically, drying is performed to the extent that themixture contains less than 2 weight percent of water. Generally, firingmay take place at a temperature of at least 1250° C., preferably between1250 and 1550° C., typically between 1300 and 1530° C., in particularbetween 1300 and 1520° C., typically for a period of up to about 8 hoursand preferably from 2 to 6 hours. Drying and firing may be carried outin any atmosphere, such as in air, nitrogen, or helium, or mixturesthereof. Preferably, in particular when the formed bodies containorganic material, the firing is at least in part or entirely carried outin an oxidizing atmosphere, such as in an oxygen-containing atmosphere.

The performance of the catalyst may be enhanced if the carrier iswashed, to remove soluble residues, before deposition of other catalystingredients on the carrier. On the other hand, unwashed carriers mayalso be used successfully. A useful method for washing the carriercomprises washing the carrier in a continuous fashion with hot,demineralised water, until the electrical conductivity of the effluentwater does not further decrease. A suitable temperature of thedemineralised water is in the range of 80 to 100° C., for example 90° C.or 95° C. Reference may be made to WO-00/15333 and U.S. Pat. No.6,368,998, which are incorporated herein by reference.

Generally, the catalyst of this invention comprises silver as acatalytically active metal. Appreciable catalytic activity is obtainedby employing a silver content of the catalyst of at least 10 g/kg, inparticular at least 50 g/kg, relative to the weight of the catalyst. Thepreparation of the catalysts is known in the art and the known methodsare applicable to the preparation of the catalyst of this invention.Methods of preparing the catalyst include impregnating the carrier witha silver compound and performing a reduction to form metallic silverparticles. Catalysts having relatively high silver content may beprepared by multiple impregnation, for example double or tripleimpregnation. Reference may be made, for example, to U.S. Pat. No.5,380,697, U.S. Pat. No. 5,739,075, U.S. Pat. No. 6,368,998,US-2002/0010094 A1, EP-A-266015, WO-00/15333, WO-00/15334 andWO-00/15335, which are incorporated herein by reference.

The impregnation may include impregnation with a solution of which thepH has a value above 12, for example 13 or 13.2 or above. This may beaccomplished by the addition of a base to the impregnation solution, forexample lithium hydroxide, cesium hydroxide or a tetraalkylammoniumhydroxide, such as tetramethylammonium hydroxide or tetraethylammoniumhydroxide, in sufficient quantity. Dependent of the composition of theimpregnation solution, a quantity of base in the range of from 20 to 70mmole/kg carrier, for example 30, 40, 50 or 60 mmole/kg carrier may besufficient to achieve a sufficiently high pH.

The reduction of cationic silver to metallic silver may be accomplishedduring a step in which the catalyst is dried, so that the reduction assuch does require a separate process step. This may be the case if theimpregnation solution comprises a reducing agent, for example, anoxalate, as described in the Examples hereinafter.

The catalyst preferably comprises silver, and a further element orcompound thereof. Eligible further elements may be selected from thegroup of nitrogen, sulfur, phosphorus, boron, fluorine, Group IA metals,Group IIA metals, rhenium, molybdenum, tungsten, chromium, titanium,hafnium, zirconium, vanadium, manganese, thallium, thorium, tantalum,niobium, gallium and germanium and mixtures thereof. Preferably theGroup IA metals are selected from lithium, potassium, rubidium andcesium. Most preferably the Group IA metal is lithium, potassium and/orcesium. Preferably the Group IIA metals are selected from calcium andbarium. Where possible, the further element may suitably be provided asan oxyanion, for example, as a sulfate, borate, perrhenate, molybdate ornitrate, in salt or acid form.

It is preferred to employ the carrier of this invention in thepreparation of a highly selective catalyst. The highly selectivesilver-based catalysts may comprise, in addition to silver, one or moreof rhenium, molybdenum, tungsten, a Group IA metal, and a nitrate- ornitrite-forming compound, which may each be present in a quantity offrom 0.01 to 500 mmole/kg, calculated as the element (rhenium,molybdenum, tungsten, the Group IA metal or nitrogen) on the totalcatalyst. The nitrate- or nitrite-forming compounds and particularselections of nitrate- or nitrite-forming compounds are as definedhereinafter. The nitrate- or nitrite-forming compound is in particular aGroup IA metal nitrate or a Group IA metal nitrite. Rhenium, molybdenum,tungsten or the nitrate- or nitrite-forming compound may suitably beprovided as an oxyanion, for example as a perrhenate, molybdate,tungstate or nitrate, in salt or acid form.

Of special preference are the highly selective catalysts which compriserhenium in addition to silver. Such catalysts are known fromEP-A-266015, U.S. Pat. No. 4,761,394 and U.S. Pat. No. 4,766,105, whichare incorporated herein by reference. Broadly, they comprise silver,rhenium or compound thereof, the further element (as definedhereinbefore, in particular tungsten, molybdenum and/or a Group IAmetal, in particular lithium and/or cesium) other than rhenium orcompound thereof, and optionally a rhenium co-promoter. The rheniumco-promoter may be selected from one or more of sulfur, phosphorus,boron, and compounds thereof.

Preferred amounts of the components of the catalysts are, whencalculated as the element, relative to the weight of the catalyst:

silver from 10 to 500 g/kg,

rhenium from 0.01 to 50 mmole/kg, if present,

the further element or elements, if present, each from 0.1 to 500mmole/kg, and,

the rhenium co-promoter from 0.1 to 30 mmole/kg, if present.

With respect to silver, this metal is present preferably in an amount of50 to 500 g/kg, more preferably 50 to 400 g/kg, in particular 50 to 350g/kg, for example 105 g/kg, or 120 g/kg, or 145 g/kg, or 191 g/kg, or200 g/kg, or 250 g/kg, or 290 g/kg, or 310 g/kg. Rhenium may preferablybe present in an amount of from 0.1 to 10 mmoles/kg, for example 2mmoles/kg, or 3 mmoles/kg, or 5 mmoles/kg. The further element orelements may each be present in a preferred amount of from 0.5 to 100mmole/kg. For example, tungsten may typically be present in an amount inthe range of from 0.5 to 20 mmole/kg, such as 1 mmole/kg, or 1.5mmoles/kg, or 5 mmole/kg, or 15 mmole/kg; molybdenum may typically bepresent in an amount in the range of from 1 to 40 mmole/kg, such as 2.3mmole/kg, or 12 mmole/kg, or 25 mmole/kg; and the alkali metal may eachtypically be present in amount of from 5 to 100 mmole/kg. Suitableamounts for lithium are for example 5 mmole/kg, or 10 mmole/kg, or 22.2mmole/kg, or 30 mmole/kg, or 40 mmole/kg, or 50 mmole/kg. Suitableamounts for cesium are for example 5 mmole/kg, or 5.3 mmole/kg, or 5.4mmole/kg, or 6.1 mmole/kg, or 6.2 mmole/kg, or 6.4 mmole/kg, or 7.2mmole/kg, or 7.5 mmole/kg, or 10 mmole/kg, or 15 mmole/kg, or 33mmole/kg, or 47 mmole/kg.

Although the present epoxidation process may be carried out in manyways, it is preferred to carry it out as a gas phase process, i.e. aprocess in which the feed is contacted in the gas phase with thecatalyst which is present as a solid material, typically in a packedbed. Generally the process is carried out as a continuous process.

The olefin for use in the present epoxidation process may be any olefin,such as an aromatic olefin, for example styrene, or a di-olefin, whetherconjugated or not, for example 1,9-decadiene or 1,3-butadiene. Mixturesof olefins may be used. Typically, the olefin is a monoolefin, forexample 2-butene or isobutene. Preferably, the olefin is amono-α-olefin, for example 1-butene or propylene. The most preferredolefin is ethylene.

The olefin concentration in the feed may be selected within a widerange. Typically, the olefin concentration in the feed will be at most80 mole percent, relative to the total feed. Preferably, it will be inthe range of from 0.5 to 70 mole percent, in particular from 1 to 60mole percent, on the same basis. As used herein, the feed is consideredto be the composition which is contacted with the catalyst.

The present epoxidation process may be air-based or oxygen-based, see“Kirk-Othmer Encyclopedia of Chemical Technology”, 3^(rd) edition,Volume 9, 1980, pp. 445-447. In the air-based process air or airenriched with oxygen is employed as the source of the oxidizing agentwhile in the oxygen-based processes high-purity (at least 95 molepercent) oxygen is employed as the source of the oxidizing agent.Presently most epoxidation plants are oxygen-based and this is apreferred embodiment of the present invention.

The oxygen concentration in the feed may be selected within a widerange. However, in practice, oxygen is generally applied at aconcentration which avoids the flammable regime. Typically, theconcentration of oxygen applied will be within the range of from 1 to 15mole percent, more typically from 2 to 12 mole percent of the totalfeed.

In order to remain outside the flammable regime, the concentration ofoxygen in the feed may be lowered as the concentration of the olefin isincreased. The actual safe operating ranges depend, along with the feedcomposition, also on the reaction conditions such as the reactiontemperature and the pressure.

A reaction modifier may be present in the feed for increasing theselectively, suppressing the undesirable oxidation of olefin or olefinoxide to carbon dioxide and water, relative to the desired formation ofolefin oxide. Many organic compounds, especially organic halides andorganic nitrogen compounds, may be employed as the reaction modifier.Nitrogen oxides, hydrazine, hydroxylamine or ammonia may be employed aswell. It is frequently considered that under the operating conditions ofolefin epoxidation the nitrogen containing reaction modifiers areprecursors of nitrates or nitrites, i.e. they are so-called nitrate- ornitrite-forming compounds (cf. e.g. EP-A-3642 and U.S. Pat. No.4,822,900, which are incorporated herein by reference).

Organic halides are the preferred reaction modifiers, in particularorganic bromides, and more in particular organic chlorides. Preferredorganic halides are chlorohydrocarbons or bromohydrocarbons. Morepreferably they are selected from the group of methyl chloride, ethylchloride, ethylene dichloride, ethylene dibromide, vinyl chloride or amixture thereof. Most preferred reaction modifiers are ethyl chlorideand ethylene dichloride.

Suitable nitrogen oxides are of the general formula NO_(x) wherein x isin the range of from 1 to 2, and include for example NO, N₂O₃ and N₂O₄.Suitable organic nitrogen compounds are nitro compounds, nitrosocompounds, amines, nitrates and nitrites, for example nitromethane,1-nitropropane or 2-nitropropane. In preferred embodiments, nitrate- ornitrite-forming compounds, e.g. nitrogen oxides and/or organic nitrogencompounds, are used together with an organic halide, in particular anorganic chloride.

The reaction modifiers are generally effective when used in lowconcentration in the feed, for example up to 0.1 mole percent, relativeto the total feed, for example from 0.01×10⁻⁴ to 0.01 mole percent. Inparticular when the olefin is ethylene, it is preferred that thereaction modifier is present in the feed at a concentration of from0.1×10⁻⁴ to 50×10⁻⁴ mole percent, in particular from 0.3×10⁻⁴ to 30×10⁻⁴mole percent, relative to the total feed.

In addition to the olefin, oxygen and the reaction modifier, the feedmay contain one or more optional components, such as carbon dioxide,inert gases and saturated hydrocarbons. Carbon dioxide is a by-productin the epoxidation process. However, carbon dioxide generally has anadverse effect on the catalyst activity. Typically, a concentration ofcarbon dioxide in the feed in excess of 25 mole percent, preferably inexcess of 10 mole percent, relative to the total feed, is avoided. Aconcentration of carbon dioxide as low as 1 mole percent or lower,relative to the total feed, may be employed. A suitable concentration ofcarbon monoxide may be in the range of from 0.2 to 0.8 mole percent, forexample 0.5 mole percent, relative to the total feed. Inert gases, forexample nitrogen or argon, may be present in the feed in a concentrationof from 30 to 90 mole percent, typically from 40 to 80 mole percent.Suitable saturated hydrocarbons are methane and ethane. If saturatedhydrocarbons are present, they may be present in a quantity of up to 80mole percent, relative to the total feed, in particular up to 75 molepercent. Frequently they are present in a quantity of at least 30 molepercent, more frequently at least 40 mole percent. Saturatedhydrocarbons may be added to the feed in order to increase the oxygenflammability limit.

The epoxidation process may be carried out using reaction temperaturesselected from a wide range. Preferably the reaction temperature is inthe range of from 150 to 325° C., more preferably in the range of from180 to 300° C.

The epoxidation process is preferably carried out at a reactor inletpressure in the range of from 1000 to 3500 kPa. “GHSV” or Gas HourlySpace Velocity is the unit volume of gas at normal temperature andpressure (0° C., 1 atm, i.e. 101.3 kPa) passing over one unit volume ofpacked catalyst per hour. Preferably, when the epoxidation process is asa gas phase process involving a packed catalyst bed, the GHSV is in therange of from 1500 to 10000 Nl/(l·h). Preferably, the process is carriedout at a work rate in the range of from 0.5 to 10 kmole olefin oxideproduced per m³ of catalyst per hour, in particular 0.7 to 8 kmoleolefin oxide produced per m³ of catalyst per hour, for example 5 kmoleolefin oxide produced per m³ of catalyst per hour. As used herein, thework rate is the amount of the olefin oxide produced per unit volume ofcatalyst per hour and the selectivity is the molar quantity of theolefin oxide formed relative to the molar quantity of the olefinconverted.

The olefin oxide produced may be recovered from the reaction mixture byusing methods known in the art, for example by absorbing the olefinoxide from a reactor outlet stream in water and optionally recoveringthe olefin oxide from the aqueous solution by distillation. At least aportion of the aqueous solution containing the olefin oxide may beapplied in a subsequent process for converting the olefin oxide into a1,2-diol or a 1,2-diol ether.

The olefin oxide produced in the epoxidation process may be convertedinto a 1,2-diol, a 1,2-diol ether, or an alkanolamine. As this inventionleads to a more attractive process for the production of the olefinoxide, it concurrently leads to a more attractive process whichcomprises producing the olefin oxide in accordance with the inventionand the subsequent use of the obtained olefin oxide in the manufactureof the 1,2-diol, 1,2-diol ether, and/or alkanolamine.

The conversion into the 1,2-diol or the 1,2-diol ether may comprise, forexample, reacting the olefin oxide with water, suitably using an acidicor a basic catalyst. For example, for making predominantly the 1,2-dioland less 1,2-diol ether, the olefin oxide may be reacted with a ten foldmolar excess of water, in a liquid phase reaction in presence of an acidcatalyst, e.g. 0.5-1 weight percent sulfuric acid, based on the totalreaction mixture, at 50-70° C. at 1 bar absolute, or in a gas phasereaction at 130-240° C. and 20-40 bar absolute, preferably in theabsence of a catalyst. If the proportion of water is lowered theproportion of 1,2-diol ethers in the reaction mixture is increased. The1,2-diol ethers thus produced may be a di-ether, tri-ether, tetra-etheror a subsequent ether. Alternative 1,2-diol ethers may be prepared byconverting the olefin oxide with an alcohol, in particular a primaryalcohol, such as methanol or ethanol, by replacing at least a portion ofthe water by the alcohol.

The conversion into the alkanolamine may comprise, for example, reactingthe olefin oxide with ammonia. Anhydrous or aqueous ammonia may be used,although anhydrous ammonia is typically used to favor the production ofmonoalkanolamine. For methods applicable in the conversion of the olefinoxide into the alkanolamine, reference may be made to, for example U.S.Pat. No. 4,845,296, which is incorporated herein by reference.

The 1,2-diol and the 1,2-diol ether may be used in a large variety ofindustrial applications, for example in the fields of food, beverages,tobacco, cosmetics, thermoplastic polymers, curable resin systems,detergents, heat transfer systems, etc. The alkanolamine may be used,for example, in the treating (“sweetening”) of natural gas.

Unless specified otherwise, the low-molecular weight organic compoundsmentioned herein, for example the olefins, 1,2-diols, 1,2-diol ethers,alkanolamines and reaction modifiers, have typically at most 40 carbonatoms, more typically at most 20 carbon atoms, in particular at most 10carbon atoms, more in particular at most 6 carbon atoms. As definedherein, ranges for numbers of carbon atoms (i.e. carbon number) includethe numbers specified for the limits of the ranges.

Having generally described the invention, a further understanding may beobtained by reference to the following examples, which are provided forpurposes of illustration only and are not intended to be limiting unlessotherwise specified.

EXAMPLES Example 1 Preparation of Carriers

A carrier (designated hereinafter as “Carrier A”) was made by mixing thefollowing ingredients:

1. 75.8 parts by weight (pbw) of an α-alumina with d₅₀ of 21 μm;

2. 20 pbw of an α-alumina with d₅₀ of 3 μm;

3. 3 pbw of boehmite, calculated as Al₂O₃;

4. 0.2 pbw of magnesium silicate, calculated as MgSiO₃; and

5. 1 pbw of zirconium oxide.

To this mixture were added 10 weight percent, relative to the mixtureweight, of petroleum jelly and 8 weight percent, relative to the mixtureweight, of starch and 0.1 weight percent, relative to the mixtureweight, of boric acid. Water was then added in an amount to make themixture extrudable and this mixture was then extruded to form formedbodies in the form of hollow cylinders that are about 6 mm in diameterand 6 mm long. These were then dried and fired in a kiln at 1480° C.,for 5 hours in air to produce Carrier A. As regards procedures followedin this carrier preparation, reference may be made to US2003/0162984-A1.

A second carrier (hereinafter “Carrier B”) was made by the sameprocedure as Carrier A, except that 75.8 parts by weight (pbw) of anα-alumina with d₅₀ of 15 μm was used instead of the α-alumina with d₅₀of 21 μm.

A third carrier (hereinafter “Carrier C”) was made by the same procedureas Carrier A, except that:

no zirconium dioxide was used;

76.8 parts by weight (pbw) of an α-alumina with d₅₀ of 15 μm was usedinstead of the α-alumina with d₅₀ of 21 μM; and

firing was carried out at 1510° C., for 5 hours, instead of at 1480° C.

For comparative purposes, a fourth carrier (hereinafter “Carrier D”) wasprepared according to the process as described for “Carrier A” in theExamples of US 2003/0162984.

For comparative purposes, a fifth carrier (hereinafter “Carrier E”) wasmade by the same procedure as Carrier A, except that:

no boehmite was used;

68.8 parts by weight (pbw) of an α-alumina with d₅₀ of 31 μm was usedinstead of the α-alumina with d₅₀ of 21 μm;

30 parts by weight (pbw) of an α-alumina with d₅₀ of 3 μm was usedinstead of 20 pbw; and

firing was carried out at 1450° C., for 5 hours, instead of at 1480° C.

The carriers exhibited characteristics as indicated in Table I. The poresize distribution is specified as the volume fraction (volume percent)of the pores having diameters in the specified ranges (<0.1 μm, 0.1-10μm, 0.1-0.3 μm, 0.2-0.3 μm, 0.3-10 μm, 5-10 μm, and >10 μm), relative tothe total pore volume. “Pore volume” represents the total pore volume.“D₅₀” represents the median pore diameter.

TABLE I Surface Water Pore Pore size distribution (volume percent) *)area absorption volume D₅₀ <0.1 0.1-10 0.1-0.3 0.2-0.3 0.3-10 >10 5-10Carrier (m²/g) (g/g) (ml/g) (μm) μm μm μm μm μm μm μm A 1.68 0.50 0.501.8 <1 94 2 2 92 6 23 B 1.72 0.50 0.50 1.0 <1 97 2 1 95 3 8 C 1.55 0.430.42 1.0 <1 97 2 2 95 3 5 D **) 2.00 0.42 0.41 2.2 <1 97 22 17 75 3 13 E**) 1.95 0.45 0.43 2.8 <1 93 23 18 70 6 25 *) volume percent relative tothe total pore volume **) comparative

Example 2 Preparation of Catalysts

A silver-amine-oxalate stock solution was prepared by the followingprocedure:

415 g of reagent-grade sodium hydroxide were dissolved in 2340 mlde-ionized water and the temperature was adjusted to 50° C.

1699 g high purity “Spectropure” silver nitrate was dissolved in 2100 mlde-ionized water and the temperature was adjusted to 50° C.

The sodium hydroxide solution was added slowly to the silver nitratesolution, with stirring, while maintaining a solution temperature of 50°C. This mixture was stirred for 15 minutes, then the temperature waslowered to 40° C.

Water was removed from the precipitate created in the mixing step andthe conductivity of the water, which contained sodium and nitrate ions,was measured. An amount of fresh deionized water equal to the amountremoved was added back to the silver solution. The solution was stirredfor 15 minutes at 40° C. The process was repeated until the conductivityof the water removed was less than 90 μmho/cm. 1500 ml fresh deionizedwater was then added.

630 g of high-purity oxalic acid dihydrate were added in approximately100 g increments. The temperature was kept at 40° C. and the pH was keptabove 7.8.

Water was removed from this mixture to leave a highly concentratedsilver-containing slurry. The silver oxalate slurry was cooled to 30° C.

699 g of 92 weight percent ethylenediamine (8% de-ionized water) wasadded while maintaining a temperature no greater than 30° C. The finalsolution was used as a stock silver impregnation solution for preparingthe catalysts.

Carriers A, B, C, D, and E, prepared according to Example 1, were usedto make silver catalysts, as follows, to form Catalyst A (according tothe invention), Catalyst B (according to the invention) Catalyst C(according to the invention), Catalyst D (for comparison), and CatalystE (for comparison), respectively. Actual silver and cesium loadings havebeen specified in Table II, hereinafter. Catalysts A, B, C, D, and Ealso contained 2.8 mmoles rhenium/kg catalyst, 12 mmoles lithium/kgcatalyst, and 0.6 mmoles tungsten/kg catalyst.

Catalyst A (According to the Invention):

Catalyst A was prepared in two impregnation steps.

To 191 grams of stock impregnation solution of specific gravity 1.548g/ml was added 13.0 grams of water, resulting in a solution with aspecific gravity of 1.496 g/ml. A vessel containing 120 grams of CarrierA was evacuated to 20 mm Hg for 1 minute and the impregnation solutionwas added to Carrier A while under vacuum, then the vacuum was releasedand the carrier allowed to contact the liquid for 3 minutes. Theimpregnated Carrier A was then centrifuged at 500 rpm for 2 minutes toremove excess liquid. Impregnated Carrier A pellets were placed in avibrating shaker and dried in air flowing at a rate of 16.2 Nl/h at 250°C. for 5.5 minutes. The resulting dried catalyst precursor containedapproximately 17.2 weight percent silver.

The dried Catalyst A Precursor was then impregnated with a secondsolution which was made by mixing 191.0 grams of silver stock solutionof specific gravity 1.548 g/ml with a solution of 0.2980 g of ammoniumperrhenate in 2 g of 1:1 (w/w) ethylenediamine/water, 0.0594 g ofammonium metatungstate dissolved in 2 g of 1:1 ammonia/water and 0.3283g lithium nitrate dissolved in water. Additional water was added toadjust the specific gravity of the solution to 1.496 g/ml. 50 grams ofsuch doped solution was mixed with 0.1830 g of 46.07 weight percentcesium hydroxide solution. This final impregnation solution was used toprepare Catalyst A. A vessel containing 30 grams of the Catalyst APrecursor was evacuated to 20 mm Hg for 1 minute and the finalimpregnation solution was added while under vacuum, then the vacuum wasreleased and the precursor allowed to contact the liquid for 3 minutes.The impregnated precursor was then centrifuged at 500 rpm for 2 minutesto remove excess liquid. Catalyst A pellets were placed in a vibratingshaker and dried in air flowing at a rate of 16.2 Nl/h at 250° C. for5.5 minutes.

Catalyst B (According to the Invention):

Catalyst B was prepared in the same manner as Catalyst A, using 120grams carrier B. The specific gravity of the impregnation solution inthe first impregnation was 1.563. The dried Catalyst B Precursor wasthen impregnated with a second solution which was made by mixing 194.0grams of silver stock solution of specific gravity 1.563 g/ml with asolution of 0.3160 g of ammonium perrhenate in 2 g of 1:1 (w/w)ethylenediamine/water, 0.0629 g of ammonium metatungstate dissolved in 2g of 1:1 ammonia/water and 0.3481 g lithium nitrate dissolved in water.Additional water was added to adjust the specific gravity of thesolution to 1.521 g/ml. The total water added was 10.0 grams. 50 gramsof such doped solution was mixed with 0.1827 g of 46.07 weight percentcesium hydroxide solution. This final impregnation solution was used toprepare Catalyst B.

Catalyst C (According to the Invention):

Catalyst C was prepared in the same manner as Catalyst A, using 120grams carrier C. The specific gravity of the impregnation solution inthe first impregnation was 1.552. The dried Catalyst C Precursor wasthen impregnated with a second solution which was made by mixing 232grams of silver stock solution of specific gravity 1.552 g/ml with asolution of 0.4077 g of ammonium perrhenate in 2 g of 1:1 (w/w)ethylenediamine/water, 0.0812 g of ammonium metatungstate dissolved in 2g of 1:1 ammonia/water and 0.4491 g lithium nitrate dissolved in water.Additional water was added to adjust the specific gravity of thesolution to 1.511 g/ml. The total water added was 11.9 grams. 50 gramsof such doped solution was mixed with 0.2534 g of 46.07 weight percentcesium hydroxide solution. This final impregnation solution was used toprepare Catalyst C.

Catalyst D (Comparative):

Catalyst D was prepared in the same manner as Catalyst A, using 120grams carrier D. The specific gravity of the impregnation solution inthe first impregnation was 1.529 g/ml. The dried Catalyst D Precursorwas then impregnated with a second solution which was made by mixing199.3 grams of silver stock solution of specific gravity 1.548 g/ml witha solution of 0.3370 g of ammonium perrhenate in 2 g of 1:1 (w/w)ethylenediamine/water, 0.0671 g of ammonium metatungstate dissolved in 2g of 1:1 ammonia/water and 0.3713 g lithium nitrate dissolved in water.Additional water was added to adjust the specific gravity of thesolution to 1.529 g/ml. The total water added was 4.7 grams. 50 grams ofsuch doped solution was mixed with 0.2435 g of 46.07 weight percentcesium hydroxide solution. This final impregnation solution was used toprepare Catalyst D.

Catalyst E (Comparative):

Catalyst E was prepared in the same manner as Catalyst A, using 120grams carrier E. The specific gravity of the impregnation solution inthe first impregnation was 1.527 g/ml. The dried Catalyst E Precursorwas then impregnated with a second solution which was made by mixing199.0 grams of silver stock solution of specific gravity 1.548 g/ml witha solution of 0.3218 g of ammonium perrhenate in 2 g of 1:1 (w/w)ethylenediamine/water, 0.0641 g of ammonium metatungstate dissolved in 2g of 1:1 ammonia/water and 0.3545 g lithium nitrate dissolved in water.Additional water was added to adjust the specific gravity of thesolution to 1.527 g/ml. The total water added was 5.0 grams. 50 grams ofsuch doped solution was mixed with 0.2093 g of 46.07 weight percentcesium hydroxide solution. This final impregnation solution was used toprepare Catalyst E.

TABLE II Silver Cesium Content Content Catalyst (mmoles/kg) % w A *) 6.429.0 B *) 6.2 30.7 C *) 6.5 27.0 D **) 7.5 26.8 E **) 6.8 28.0 *)invention **) comparative

Example 3 Testing of Catalysts

The catalysts were used to produce ethylene oxide from ethylene andoxygen. To do this, crushed catalyst were loaded into a stainless steelU-shaped tube. The tube was immersed in a molten metal bath (heatmedium) and the ends were connected to a gas flow system. The weight ofcatalyst used and the inlet gas flow rate (0.28 Nl/minute) were adjustedto give a gas hourly space velocity of 33001 Nl/(l·h), as calculated foruncrushed catalyst. The inlet gas pressure was 1550 kPa (absolute).

The gas mixture passed through the catalyst bed, in a “once-through”operation, during the entire test run including the start-up, consistedof 30.0 volume percent ethylene, 8.0 volume percent oxygen, 5.0 volumepercent carbon dioxide, 57 volume percent nitrogen and 1.0 to 6.0 partsper million by volume (ppmv) ethyl chloride.

The initial reactor temperature was 180° C., and this was ramped up at arate of 10° C. per hour to 225° C. and then adjusted so as to achieve aconstant ethylene oxide content of 3.1 volume percent in the outlet gasstream at an ethyl chloride concentration of 1.3 ppmv. Performance dataat this conversion level are usually obtained for initial peakselectivity. Depending upon the catalyst used and the parameters of theolefin epoxidation process, the time required to reach the initial, peakselectivity, that is the highest selectivity reached in the initialstage of the process, may vary. For example, the initial, peakselectivity of a process may be achieved after only 1 or 2 days ofoperation or may be achieved after as much as, for example, 1 month ofoperation. In the testing of Catalysts A, B, C, D, and E, the activityand selectivity were also measured upon continued operation. The resultsobtained after a cumulative production of ethylene oxide of 0.5 kton/m³and 1 kton/m³ of catalyst are also reported in Table III.

An advantage of the present invention is that catalysts made accordingto this invention exhibit increased initial selectivity at the sameethylene oxide production levels. Also, the present invention exhibitsimproved stability.

TABLE III Selectivity Temperature Catalyst (%) (° C.) A *), initially88.0 247 at 0.5 kton/m³ 87.6 253 at 1 kton/m³ 86.2 257 B *), initially87.6 251 at 0.5 kton/m³ 87.6 253 at 1 kton/m³ 86.1 263 C *), initially89.1 251 at 0.5 kton/m³ 88.7 254 at 1 kton/m³ 87.9 257 D **), initially85.7 247 at 0.5 kton/m³ 84.8 251 at 1 kton/m³ 83.5 255 E **), initially86.8 255 at 0.5 kton/m³ 85.2 257 at 1 kton/m³ 82.7 267 *) invention **)comparative

1. A process for the epoxidation of an olefin, which process comprisesreacting a feed comprising an olefin and oxygen in the presence of acatalyst comprising a carrier and silver deposited on the carrier, whichcarrier comprises at least 85 weight percent α-alumina and has a surfacearea of at least 1.3 m²/g, a median pore diameter of more than 0.8 μm,and a pore size distribution wherein at least 80% of the total porevolume is contained in pores with diameters in the range of from 0.1 to10 μm and at least 80% of the pore volume contained in the pores withdiameters in the range of from 0.1 to 10 μm is contained in pores withdiameters in the range of from 0.3 to 10 μm.
 2. A process as claimed inclaim 1, wherein the olefin is ethylene.
 3. A process as claimed inclaim 1, wherein the feed further comprises a reaction modifier selectedfrom the group consisting of organic halides, organic nitrogencompounds, nitrogen oxides, hydrazine, hydroxylamine and ammonia.
 4. Aprocess as claimed in claim 1, wherein the feed additionally comprisescarbon dioxide in a concentration of above 1 mole percent, relative tothe total feed.
 5. A process as claimed in claim 1, wherein the feedadditionally comprises carbon dioxide in a concentration of 1 molepercent or lower, relative to the total feed.
 6. A process as claimed inclaim 1, wherein an olefin oxide is formed at a work rate in the rangeof from 0.5 to 10 kmole olefin oxide per m³ of catalyst per hour.
 7. Aprocess as claimed in claim 1 wherein the carrier has a pore sizedistribution such that at least 90% of the total pore volume iscontained in the pores with diameters in the range of from 0.1 to 10 μm;at least 90% of the pore volume contained in the pores with diameters inthe range of from 0.1 to 10 μm is contained in pores with diameters inthe range of from 0.3 to 10 μm; the pores with diameters greater than 10μm represent less than 10% of the total pore volume; and the pores withdiameters less than 0.3 μm represent less than 10% of the total porevolume.
 8. A process as claimed in claim 7 wherein the pores withdiameters greater than 10 μm represent at most 8% of the total porevolume; and the pores with diameters less than 0.3 μm represent at most3% of the total pore volume.
 9. A process as claimed in claim 1 whereinthe carrier has a median pore diameter of at most 2 μm, a total porevolume in the range of from 0.25 to 0.8 ml/g and a surface area of atmost 5 m²/g, a total pore volume in the range of from 0.3 to 0.7 ml/gand a surface area in the range of from 1.3 to 3 m²/g.
 10. A process asclaimed in claim 1 wherein the carrier has a total pore volume of atmost 0.6 ml/g.
 11. A process as claimed in claim 1 wherein the carriercomprises at least 95 weight percent α-alumina and the carrier has amedian pore diameter in the range of from 0.9 to 1.8 μm, a waterabsorption in the range of from 0.3 to 0.7 g/g and a surface area in therange of from 1.4 m²/g to 2.5 m²/g.
 12. A process as claimed in claim 1wherein the carrier has a water absorption of at most 0.6 g/g.
 13. Aprocess as claimed in claim 1 wherein silver is deposited on the carrierin a quantity of from 10 to 500 g/kg, relative to the weight of thecatalyst.
 14. A process as claimed in claim 1 wherein the carriercomprises alumina and a bond material.
 15. A process as claimed in claim14 wherein the carrier has an α-alumina content of at least 95 weightpercent, and wherein the bond material comprises an alkaline earth metalsilicate bond material.
 16. A process as claimed in claim 15 wherein thebond material further comprises one or more additional componentsselected from the group consisting of a hydrated alumina, a zirconiumcomponent and a titanium component.
 17. A process as claimed in claim 1which comprises, deposited on the carrier in addition to silver, one ormore further elements selected from the group consisting of nitrogen,sulfur, phosphorus, boron, fluorine, Group IA metals, Group IIA metals,rhenium, molybdenum, tungsten, chromium, titanium, hafnium, zirconium,vanadium, manganese, thallium, thorium, tantalum, niobium, gallium andgermanium and mixtures thereof.
 18. A process as claimed in claim 17wherein the Group IA metals are selected from the group consisting oflithium, potassium, rubidium and cesium.
 19. A process as claimed inclaim 1 which comprises, deposited on the carrier in addition to silver,one or more of rhenium, molybdenum, tungsten, Group IA metals, andnitrate- or nitrite-forming compounds.
 20. A process as claimed in claim1 which comprises rhenium deposited on the carrier in addition tosilver, and optionally a rhenium co-promoter selected from the groupconsisting of sulfur, phosphorus, boron, and compounds thereof.
 21. Aprocess as claimed in claim 1 wherein the carrier has a pore sizedistribution such that at least 75%, the pore volume contained in poreswith diameters in the range of from 0.1 to 10 μm is contained in poreswith diameters in the range of from 0.4 to 10 μm.
 22. A process asclaimed in claim 1 wherein the carrier has a pore size distribution suchthat the pores with diameters in the range of from 0.1 to 10 μmrepresent more than 90% of the total pore volume and at least 15% of thepore volume contained in pores with diameters in the range of from 0.1to 10 μm is contained in pores with diameters in the range of from 5 to10 μm.
 23. A process as claimed in claim 1 wherein the carrier is in theform of formed bodies fired at a temperature of at least 1250° C.
 24. Aprocess for the epoxidation of an olefin, which process comprisesreacting a feed comprising an olefin and oxygen in the presence of acatalyst comprising a carrier and silver deposited on the carrier, whichcarrier comprises a bond material and at least 85 weight percentα-alumina, and has a surface area of at least 1 m²/g, a median porediameter of more than 0.8 μm, and a pore size distribution wherein atleast 80% of the total pore volume is contained in pores with diametersin the range of from 0.1 to 10 μm and at least 80% of the pore volumecontained in the pores with diameters in the range of from 0.1 to 10 μmis contained in pores with diameters in the range of from 0.3 to 10 μm.25. A process as claimed in claim 24 wherein the surface area is atleast 1.3 m²/g.
 26. A process for the epoxidation of an olefin, whichprocess comprises reacting a feed comprising an olefin and oxygen in thepresence of a catalyst comprising a carrier and silver deposited on thecarrier, which carrier is in the form of formed bodies fired at atemperature of at least 1250° C. comprising at least 85 weight percentα-alumina, and has a surface area of at least 1 m²/g, a median porediameter of more than 0.8 μm, and a pore size distribution wherein atleast 80% of the total pore volume is contained in pores with diametersin the range of from 0.1 to 10 μm and at least 80% of the pore volumecontained in the pores with diameters in the range of from 0.1 to 10 μmis contained in pores with diameters in the range of from 0.3 to 10 μm.27. A process for the epoxidation of an olefin, which process comprisesreacting a feed comprising an olefin and oxygen in the presence of acatalyst comprising a carrier and silver deposited on the carrier, whichcarrier comprises at least 85 weight percent α-alumina and has anon-platelet morphology, a surface area of at least 1 m²/g, a medianpore diameter of more than 0.8 μm, and a pore size distribution whereinat least 80% of the total pore volume is contained in pores withdiameters in the range of from 0.1 to 10 μm and at least 80% of the porevolume contained in the pores with diameters in the range of from 0.1 to10 μm is contained in pores with diameters in the range of from 0.3 to10 μm.
 28. A process for the epoxidation of an olefin, which processcomprises reacting a feed comprising an olefin and oxygen in thepresence of a catalyst comprising a carrier and silver deposited on thecarrier, which carrier has a surface area of at least 1 m²/g, a waterabsorption of at most 0.6 g/g, a median pore diameter of more than 0.8μm, and a pore size distribution wherein at least 80% of the total porevolume is contained in pores with diameters in the range of from 0.1 to10 μm and at least 80% of the pore volume contained in the pores withdiameters in the range of from 0.1 to 10 μm is contained in pores withdiameters in the range of from 0.3 to 10 μm.